Ion Transport Membranes (ITMs) are ceramic membranes that are useful in gas separation such as the production of oxygen and hydrogen gases from gas streams. In ITMs, the separation of gases is based on ion conduction, where particular gases may be selectively transported across the ceramic material in the form of ionic flux. Ceramic membranes may also enable separations based on mixed ionic and electronic conduction or mixed proton-electron conductivity. Most oxygen ion transport membrane materials only conduct oxygen at temperatures above 700° C. (973 K), but despite the need for the membrane to be heated, the energy required is significantly less than for other forms of oxygen production. Accordingly, ITMs have attracted considerable research interest because they are able to separate oxygen from air and produce oxygen at lower costs than those incurred using the conventional cryogenic distillation process.
One problem associated with the use of ITMs is that oxygen permeation through conventional ITMs is slow, which limits the rate of the overall oxygen separation process. Oxygen exchange at the membrane surface and bulk diffusion within the dense membrane are considered to be the major rate-limiting steps of oxygen permeation through ITMs, and overall permeation is often jointly controlled by both processes.
In seeking to improve oxygen permeation through ITMs, some research has concentrated on modifying membrane surfaces in order to improve the oxygen surface exchange by applying catalysts. However, the absolute improvement of oxygen permeation is limited by the bulk diffusion resistance.
During the bulk diffusion-limiting process, oxygen permeation flux (OPF, Jo2) is inversely proportional to membrane thickness. Accordingly, some improvement in oxygen permeation flux can be achieved by reducing membrane thickness. However, there are limitations on the thickness of the membrane in order to ensure adequate mechanical strength. For example, disc membranes, such as those used in laboratory experiments, typically have a thickness of about 1 mm to ensure adequate mechanical strength, with the consequence that there is a long oxygen bulk diffusion distance.
Ceramic membranes with thin dense layers (which are less than 100 μm thick) that are supported on porous substrates have been developed in an effort to improve OPF by reducing the oxygen ion diffusion distance. In the design of supported membranes, the match of thermal expansion and chemical compatibility between the dense layer and the porous support needs to be carefully considered because of the high sintering and operating temperatures used (up to 1250° C.). Furthermore, the asymmetric membranes need to be sintered at high temperatures to obtain the dense layers, leading to a porous support with low porosity and isolated pores causing gas diffusion resistance in the supports. In addition, the low porosity of such supports makes it difficult to deposit catalysts for oxygen surface exchange at the interface between porous supports and dense layers.
Tubular membranes prepared by paste extrusion also suffer from a thick dense layer similar to that observed in plate membranes. However, hollow fibre membranes having outer diameters of less than 3 mm are attractive as it is possible to achieve a membrane thickness of less than 500 μm. Hollow fibre membranes are prepared by a phase inversion process which involves spinning the ceramic slurry into coagulants. Phase inversion starts from both sides of the membrane wall, caused by the internal and external coagulants. The resulting hollow fibre membrane structure consists of skin layers on two sides and a central layer sandwiched by two groups of finger-like pores.
After sintering, the central layer and the two skin layers form dense layers (i.e. three dense layers are formed), across which oxygen separation via ion transport is performed. However, the porous structure having multiple dense layers is unfavourable for oxygen permeation because permeation through the entire membrane thickness will involve oxygen exchange processes at each of the three dense layer surfaces before oxygen permeation across the hollow fibre membrane is complete. Accordingly, this limits the rate of the overall oxygen permeation process.
Two methods have been proposed to tackle the problems associated with oxygen permeation across hollow fibre membranes. In one method, acids are used to erode the skin layers and open the finger-like pores, leaving only one densified layer in the centre of the walls. Alternatively, a certain amount of solvent is added to the internal coagulant to prevent the formation of an inner skin layer or to dissolve any newly-formed skin layer. Nevertheless, both methods suffer from complicated processes for preparation of the hollow fibre membranes and/or low mechanical strength due to ultra-thin fibre walls combined with high porosity. While it is difficult to compare the performance of different types of membranes (e.g. plate versus hollow fibre) due to the different dimensions, hollow fibre membranes can obtain relatively high oxygen permeation flux (OPF) as a result of having a very thin wall. However, despite their performance, hollow fibre membranes have limited applications as they are fragile and can easily break. In addition, hollow fibre membranes cannot be readily scaled up to commercial applications, and as the hollow fibres are far from straight, they cannot be packed very densely. Accordingly, it is very difficult to assemble these fibres into, for example, a reactor in practical applications.
ITMs are also used to selectively transport ions other than O2−, for example H+ and Na+. Conventional separation membranes that rely on narrow pore sizes to achieve selective permeation of species can advantageously be supported on appropriate microchanneled articles.
Ceramic materials such as those described above have the potential for application in articles other than ceramic membranes. For example, ceramic articles may be configured for use in micro reactors. New developments in this field may identify further applications for ceramic articles that have not yet been identified.
There is a therefore a need for alternative or improved ceramic articles, such as ceramic membranes, including methods of fabricating the ceramic membranes.